This invention relates to a method of filling pressurized gas into cylinder devices, particularly applicable to hydraulic dampers or gas springs.
For filling gas under pressure into cylinder devices such as hydraulic dampers or gas springs there have been proposed various techniques, drilling a gas filling hole in the wall of the cylinder device, filling the gas into the cylinder through the hole and, thereafter, plugging the hole; and such as, as shown in U.S. Pat. No. 4,114,866, pressing a seal member inwardly against the force of a spring, thereby forming a gas filling passage between the seal member and the cylinder, filling the gas under pressure into the cylinder through the gas filling passage, and releasing the pressing force applied from the outside to outwardly displace the seal member thereby closing the gas filling passage. There are shortcomings in the former technique in that it is troublesome to drill the gas filling hole, that it is necessary to weld the gas filling hole after inserting the plug into the hole, and that it is not possible to adjust the pressure of the gas being filled or to refill the gas. In the latter technique, the construction of a gas filling apparatus incorporating a pressing member for pressing the seal member is complicated and, further, the pressing member must be actuated simultaneously with the supply of the gas under pressure, and thus the operational procedure is complicated and troublesome. Further, it has been proposed as an improved procedure in the latter technique, to omit the pressing member by utilizing the pressure of the gas being filled for inwardly displacing the seal member, thereby forming a gas filling passage along the outer surface of the seal member. However, the outer surface of the seal member usually is engaged with the inner surface of the cylinder or of a cap member closing one end of the cylinder by the force of a spring and, in the initial condition, the pressure of the gas will act on a very limited annular area of the seal member defined between the outer diameter of the piston rod and the inner diameter of the central opening of the cap member passing therethrough the piston rod and, therefore, it is difficult to inwardly displace the seal member without applying a high pressure.